The trend for semiconductor devices is smaller integrated circuit (IC) devices (also referred to as chips), packaged in smaller packages (which protect the chip while providing off chip signaling connectivity). One example are image sensors, which are IC devices that include photo-detectors which transform incident light into electrical signals (that accurately reflect the intensity and color information of the incident light with good spatial resolution). Image sensors can be front side illuminated (FSI) or back side illuminated (BSI).
A conventional front side illuminated (FSI) image sensor has photo-detectors formed at the surface of silicon chip at which light being imaged is incident. The supporting circuitry for the photo-detectors is formed over the photo-detectors, where apertures (i.e. light pipes) allow light to pass through the circuitry layers to reach the photo-detectors. The color filters and micro-lens are disposed over the surface containing the photo-detectors. The drawback with FSI image sensors is that the circuitry layers limit the size of the aperture through which incident light for each pixel must travel. As pixel size shrinks due to demands for higher numbers of pixels and smaller chip sizes, the ratio of pixel area to the overall sensor area decreases. This reduces the quantum efficiency (QE) of the sensor.
A conventional back side illuminated (BSI) image sensor is similar to an FSI image sensor, except the photo-detectors receive light through the back surface of the chip (i.e. the light enters the back surface of the chip, and travels through the silicon substrate until it reaches the photo-detectors). The color filters and micro-lens are mounted to the back surface of the chip. With this configuration, the incident light avoids the circuitry layers. However, the drawbacks with BSI image sensors include pixel cross-talk caused by diffusion in the silicon substrate (i.e. there is no circuitry or other structure that forms apertured openings to segregate the propagating light for each pixel—blue light is especially susceptible to this diffusion phenomenon) and the need for a thicker micro-lens due to shorter optical paths.
Another significant issue with BSI image sensors is that the quantum efficiency of different colors of light passing through the silicon substrate varies because the amount of the light absorbed (i.e. attenuated) by the silicon varies based upon wavelength. This means that with a uniform thickness silicon substrate, the amount of absorption of red, green and blue colors headed for the photo-detectors is not the same. In order to equalize attenuation, the different colors would have to pass through different thicknesses of the silicon. The absorption coefficients for silicon, and thickness ratios of silicon for equalizing attenuation, are provided in the table below for three different colors of light:
TABLE 1ExemplaryAbsorptionWavelengthcoefficientColor(nm)(1/cm)Thickness ratioBlue47516,0001.00Green51097001.65Red65028105.70From the above, as an example, a silicon thickness of 1 μm for blue, 1.65 μm for green and 5.70 μm for red would yield a uniform absorption for all three color wavelengths. Another measure of absorption is “absorption depth,” which is the thickness of the substrate at which about 64% (1−1/e) of the original intensity is absorbed, and about 36% (1/e) gets through. The table shows that a silicon thickness of 0.625 μm for the blue, 1.03 μm for the green and 3.56 μm for the red would yield a uniform absorption of about 64%, with 36% of the light making it through the silicon.
Any color, within limits, may represented by a linear combination of three additive primary colors, such as red, green, and blue. To enable a sensor array to sense color, a color filter array having red, green, and blue filter elements is overlaid on the sensor array so that each filter element of the color filter array is aligned with one photo detector of the sensor array (i.e. each photo detector and its associated color filter form a pixel of the image being captured). The red filter elements block green and blue light, and allow only red light to reach the corresponding photo detectors of the sensor array, which therefore output only red color components of the image. The green filter elements block red and blue light, and allow only green light to reach the corresponding photo detectors of sensor array, which therefore output only green color components of the image. The blue filter elements block red and green light, and allow only blue light to reach the corresponding photo detectors of the sensor array, which therefore output only blue color components of the image. Thus, two-thirds of the light incident on the color filter array is blocked from reaching the sensor array, significantly reducing the overall detection sensitivity of the sensor array for a color image, thereby significantly reducing the resolution of the sensor array for a color image.
A common method to capture color information in cameras using semiconductor based image sensors is to employee a mosaic, such as a Bayer pattern, of alternating red, green, and blue pixels. The light reaching these pixels is filtered by corresponding red, green, or blue light filter films made out of materials such as polyimide.
Unfortunately, as the pixel size becomes smaller, some of the light is not focused onto the photo detectors, which causes light to be lost and pixel response to decrease. The chief ray is the ray which passes through the center of an entrance pupil, and light near the center of the entrance pupil enters the pixels at the center of the array. The angle of the chief ray is commonly called the chief ray angle (CRA). At the peripheral edge of the sensing array, the pixel response drops below a certain percentage (e.g., 80%) of its zero degree angle response (where the pixel is perpendicular to incident light). The light incident near the center axis of sensor array enters the pixels near parallel to the center axis. However light incident further from the center axis enters the pixels at angles not parallel to the center axis. As a result, there can be crosstalk between the pixels that are located away from the center of the array. Crosstalk creates noise in the image sensor.
There is a need for an improved BSI image sensor configuration to make absorption of incident light through the silicon substrate substantially uniform for multiple wavelengths. There is also a need for an improved package and packaging technique for BSI image sensor chips that can provide a low profile wafer level packaging solution that is cost effective and reliable (i.e. provides the requisite mechanical support and electrical connectivity), which means that packaging solution will need to be able to integrate front end and back end processes. There is also a need for an image sensor that optimizes and improves the image quality by increasing quantum efficiency and reducing pixel crosstalk.